Understanding low flow velocities is an important challenge in characterizing the fate and transport of contaminants in wetlands, stormwater ponds, streams, coastal bays/estuaries and groundwater environments. By capturing the velocity with the aid of various sensors, progress has been made in calculating drag coefficients, diffusivity and turbulence as well as understanding the transport of nutrients and other contaminants. Furthermore, measuring velocities in heterogeneous field environments, such as constructed wetlands with multiple vegetation types, helps deepen the knowledge of the effects wind, stage and precipitation may have on the fluid motions of the system, thereby ultimately leading to better system planning, design and operation to promote efficiency in pollutant removal. In many instances these environments are dominated by low range velocities regimes (<5 cm·sec−1) characteristic of directionally variable flows and shallow depths (<15 cm), making field velocity measurements difficult without the use of expensive equipment. The development of techniques that better characterize such local-scale temporal and spatial variations in velocity and direction are particularly valuable.
Currently, few technologies exist capable of measuring the low heterogeneous velocities commonly found in wetlands and Stormwater Treatment Areas (STAs). Inexpensive current meters utilizing mechanical propellers have been found useful at higher flow rates, but begin to become infeasible at flows less than 6 cm·sec−1. Acoustic Doppler Velocimeters (ADV) and Acoustic Doppler Current Profilers (ADCP) have found popularity in wetland applications and have been shown to measure down to 0.02 cm·sec−1 in the Florida Everglades; however, they come with inherent drawbacks including depth limitations, single point volume locations, Doppler noise, signal aliasing and high costs. ADCPs have been found to operate effectively to produce current profiles in oceans, canals and rivers, but become infeasible in shallow water applications due to side lobe interference and blanking distances. With few devices available on the market, there exists a need for a cost-effective, reliable alternative for capturing low flow, directionally variable velocities.
Wetland and estuary marshes provide an important role for environmental ecosystems. The dense vegetation commonly found in these environments serve to facilitate nutrient cycling, enhance sedimentation of suspended solids, create barriers for storm surge protection and provide protective habitats for aquatic species. Through hydrodynamic processes, wetlands and marshes have been shown as effective treatment methods in reducing several pollutants from surface waters including phosphorus, suspended solids and metals. Several studies have been conducted in these environments to further characterize various fluid motion properties.
Flow velocities in the Florida Everglades have generally been recorded within a 0.0 to 3.5 cm·sec−1 range, while flow velocities in estuaries have been recorded as ranging from 0 to 28 cm·sec−1. Velocities within wetlands and marshes typically follow a spatially heterogeneous nature which may have several explanationis. The velocity heterogeneity is due to both stem-scale dispersive effects caused by velocity depressions just downstream of vegetative stems, as well as depth-scale shear dispersion effects. In addition, with sufficient low velocities and shallow water depths, wind effects may also play a role in velocity direction. Due to these factors, the velocity fields within wetlands and estuaries may vary widely on a temporal, vertical and lateral scale, and any device used to measure them should be able to account for fluctuations in both direction and magnitude. As a result wetlands have been shown to be challenging environments for velocity measurements.
Acoustic Doppler technology has been shown to work effectively in wetlands; however, it comes with some limitations. ADVs operate by measuring the Doppler shift produced when an acoustic pulse is reflected off of suspended particles moving in the water. For ADVs, the sampling volume is a “single point” approximately 0.25 cm3 located about 7 cm from the sensor, whereas for ADCPs several sampling volumes are produced sometimes measuring hundreds of meters in length from the device. Although ADVs are robust and capable of high sampling frequencies, several researchers have reported issues while operating in field environments including high levels of noise and spikes in the velocity components, signal interferences caused by velocity shear and boundary proximity and disturbances from other Doppler signals or passing boats. Obviously, ADVs have a limitation in measuring a single point, making velocity profiling only possible by physically moving the device to specific height increments. The ADCP is excellent at vertical velocity profiling; however, it becomes ineffective in shallow waters due to blanking distances and side lobe interferences. The ADCP may also be used for horizontal profiling; however, this application is limited to deeper waters such as canals and lakes.
In addition, some studies have shown difficulties in producing precise measurements for regions of wetlands. In establishing dispersive properties in the Everglades, an attempt was made to use an ADV to establish velocity profiles along a 4.8 meter stretch, but it was found that the device was insufficient due to lateral changes in the vertical velocity profile. With such heterogeneous natures, clearly single point measurements or single vertical profiles are not sufficient to fully characterize velocity fluid motions in wetlands and estuary environments, and without sufficient data, conclusions can only be made on a broad scale.
Interactions between groundwater and surface water may play a significant role in the fields of subsurface ecology, biogeochemistry, sediment quality, solute transport and remediation. Proper knowledge of water flux rates at the interface between groundwater and surface water is key in understanding and developing remediation techniques for ground or surface water treatment. Traditional methods for calculating fluxes have relied on calculations using Darcy's Law; however, they are heavily dependent on assumptions made including soil type, permeability rates and hydraulic gradients. In response, several devices have been developed to capture groundwater velocities at centimeter-scales including the Heat Pulse Flow Meters (HPFM), Point Velocity Probe (PVP), and Passive Flux Meter (PFM).
Heat Pulse Meters (HPMs) have been used for more than 20 years in estimating groundwater velocities. HPMs estimate groundwater velocity and direction by measuring the arrival and decay of a heated plume as it travels through the subsurface environment by use of thermistors surrounded by glass beads. While HPMs have been found to give accurate estimates of groundwater velocities careful attention is required to ensure proper drilling methods, well and annular size and probe placement are used. PFMs are relatively new devices which operate by installing a nylon mesh tube filled with a sorbent media and tracer mixture into a well. As groundwater flows through the media, the tracer de-sorbs at a rate proportional to the groundwater flux. Because the tracer concentrations cannot be measured until the media is pulled from the well, the method only produces time-averaged flux rates. PVPs operate by injection of a saline solution out the side of a PVC tube installed in the in-situ soil. Conductivity detectors are placed on the side of the PVC tube to measure the passing pulse (conductivity measurements over time), thereby creating a tracer curve. Using equations for flow around a cylinder, the velocity measurements are imperially derived from the tracer curve. The PVP has been shown to be effective; however, it is limited to sandy soil types and requires careful installation procedures.